For this question, I'm using the magic system discussed here that I will sum up with this: you take energy from your environment (e.g. heat from the atmosphere) and transform it into another form to cast a variety of spells. The spell itself must be either pure energy or resulting from said energy.

One particular spell is hard light, which is the purpose of this question. However, I'm having a hard time determining how I could generate hard light in compliance with the rules of my universe.

So here is what hard light is and isn't:

  • It is solid. Not necessarily in the physical sense of the word, but hard light collides with any physical object, include other hard light objects.
  • It glows. It generates faint light (as opposed to reflecting it).
  • It is cold. It isn't a freezing hazard though.
  • It does block what a wall would block. Besides, physical objects, this includes energy, e.g. heat.
  • It doesn't block what a wall wouldn't block. E.g. radio signals.
  • It only causes blunt force trauma, much unlike a light saber.
  • It is a constant-cast spell. If the caster stops casting, hard light dissipates. Ideally, does not persist, though it could take a few seconds to dissipate.
  • It is moderately translucent, as in opaque glass translucent.
  • It is not a block of ice. That would be just too easy.
  • It is not solid light, unless solid light actually exists and would meet other requirements.

I've noted in italic requirements you can play with if you have to.

The rules of magic here are:

  • Assume magic always works as advertised.
  • Assume the caster is not hurt by their spells.
  • Assume there is always just the right amount of energy to do what you want to do.
  • You can only transform energy. As such, a spell is energy, a discharge of energy or anything energy-related.
  • Outside of the aforementioned magic, assume rules of physics apply.

Given all that, what could be the physical nature of hard light?

  • 2
    $\begingroup$ What do you mean by "It radiates cold"? An object can radiate heat in the form of thermal radiation (photons), but it can't radiate the opposite. $\endgroup$
    – HDE 226868
    Apr 17, 2016 at 19:54
  • $\begingroup$ I mean it's cold, like when you put something out of the freezer. "Radiating" had a nice ring to it, but I can work with another word for it. $\endgroup$ Apr 17, 2016 at 20:18
  • $\begingroup$ Ah, okay. Post-edit, it makes sense; thank you. $\endgroup$
    – HDE 226868
    Apr 17, 2016 at 20:53
  • 3
    $\begingroup$ Seemingly related: Realism of Solid light? $\endgroup$
    – user
    Apr 17, 2016 at 20:56

5 Answers 5


Hard light could be pseudo-matter made up of 'virtual atoms' - electron shells without a nucleus. This could be done by controlling electromagnetic fields. Such material could simulate most properties of matter, such as specific heat, hardness, and transparency, but would be almost massless.

Things glow when their electrons drop to a lowered energy state. Presumably keeping the electrons in place requires a constant input of magical energy; when you funnel too much energy into the construct by accident you need to radiate it off as light to keep the construct from dissipating. Perhaps the glow of such a construct could indicate the fine control skill of the mage creating it; glow implies wasted energy (this will be the case no matter how you explain it). A truly skilled construct will have a glassy appearance with very little glow, but even a talented mage might have difficulty controlling their output when they are forced to channel more energy than they are accustomed to.

The construct could be as cold or as hot as you like, but cold implies that the virtual atoms are held firmly in place and the construct is stable, while heat suggests that it is starting to break up.

  • $\begingroup$ Wouldn't Hard Light (if we used your description) have a negative charge? or are electrons and electron shells two seperate things? $\endgroup$ Jan 15, 2017 at 18:40
  • $\begingroup$ @AnAspiringAuthor Matter made up of electrons without protons would have a negative charge, but seeing as how this is the reason why matter made up of electrons doesn't exist (electrons without protons would simply repel each other and fly apart) and we are already handwaving that away with magic, it is as likely as not that the same magic will cancel out the charge as well. For example, "real" electrons can be held in position using "virtual" magic protons that have a positive charge but no mass. $\endgroup$ Jan 16, 2017 at 6:58

If I can take the liberty of turning 'it radiates cold', which doesn't make thermodynamic sense (coldness isn't a physical property, just the absence of heat, which is), into 'it absorbs heat', which is presumably what you mean anyway.

Postulating hard light as a 'portal' with a membrane that only allows massless particles through. Or glass or somesuch, which allows you to set how translucent it is. Then further saying that it has a dim light source at the centre. In other words, so far it's a light source created from transdimensional photons with a translucent pane. This makes everything except for the heat bit work.

So, to fix this: have the membrane full of micropores. Say, slightly bigger than a water droplet (adjust to taste), then have a vacuum of the extremely cold outer space variety between the transdimensional light source and the membrane. Things like hot air currents etc. would get absorbed by this. The only issue left would be infra-red radiation, which could be solved, if desired, by the light source being a crystal that absorbs infra-red frequencies (condensed matter physics allows this). When I say 'absorb', I mean a reverse Unbestanum process. Precise regulation of number of micropores and heat of the source would hopefully be enough to not freeze things and for reasonable pressure regulation. Previously warm molecules then come out again as cold molecules having lost energy, which also helps prevent pressure issues.


To clarify, the cooling by convection doesn't work because the vacuum is cold, but because it is a vacuum: relatively small pressure inside causes any particles that fit through the micropores to go through and get absorbed by the reverse Unbestanum process. This includes warm air molecules. As already mentioned, these then get emitted as cold molecules, which also allows for finer control of the temperature and pressure.

Also worth mentioning is that when the process of constant molecule absorption stops, the membrane will automatically collapse inwards.

  • $\begingroup$ Is this membrane some sort of container for hard light then? $\endgroup$ Apr 17, 2016 at 20:29
  • $\begingroup$ "have a vacuum of the extremely cold outer space variety" Actually, in a sense, outer space isn't very cold because there is no medium in which convection can occur, so there is very limited heat transfer to the surrounding environment. This is a very real problem for spacecraft designers: it's quite difficult to get rid of waste heat in space. $\endgroup$
    – user
    Apr 17, 2016 at 20:58
  • $\begingroup$ If you call the transdimensional crystal with light source as 'hard light', then yes. Although it's not needed, it allows for the cooling thing to work without turning the crystal into what's essentially an ice block. Alternatively, you could call the whole system 'hard light'. Up to you really $\endgroup$ Apr 17, 2016 at 20:58
  • $\begingroup$ Edited to correct for my misinterpretation of how the transdimensional energy transfer works $\endgroup$ Apr 17, 2016 at 22:08
  • $\begingroup$ Oddly, the only hits from Google on Unbestanum are two posts here. So what is that? $\endgroup$
    – JDługosz
    Sep 4, 2017 at 23:09

Your rules preclude your simply conjuring up matter where there was before. However, it doesn't have to have anything to do with light/photons; it might be called that because of its appearance.

Maybe you can cause the gas and any suspended dust particles to be locked in place. Air would be as solid as a wall if you couldn't move past it as it doesn't flow around you.

It's cold because the continuing spell prevents molecules from moving: thermal energy transferred to it would normally make the molecules move faster, and the spell dampens that. Thus, it serves as a cold sink, which constantly drains the spell.

For whatever reason it's cold, you can run a sterling engine off of it, thus it's a source of power. I expect this will require energy to keep it going and/or make it run out.

  • $\begingroup$ Yeah this was where I was going to go as a first port of call, "hard light" is in fact frozen air. Peter F. Hamilton does a good treatment of the physics in the Commonwealth Saga, it could glow and absorb a certain amount of ambient energy too. The physics will be similar even if you hardwaive the cause as "magic". $\endgroup$
    – Ash
    Sep 4, 2017 at 19:04

So the spell has to be pure energy or the result of a pure energy event.

Scientifically we've been able to slow light down to a crawl and even stop it.
Using a pure energy magical field you could make the same effect, manipulating energy in an area to the point that light almost stops, though some does pass through and out of the field eventually which causes a glow.

The one problem is the blunt force trauma part, since photons are massless, and even held together in a field may behave more like a gas than a solid. However if the magical field is slowing down an area of spacetime enough to slow light, it might effect the other things in the area equally, so solidified reality.

Also you can say that once the field is established that it's self contained and while air can flow around it, anything solid like a wall, head, etc. would be excluded from the area until the casting stops.

Another possible idea is if you started the field inside of a solid object, so that the field would anchor the object to reality. Could be used as climbing anchors that don't leave marks once the spell finishes, or to seal a door so there is no way to open it as long as the spell is working.


I take it you want a plausible "physical" explanation for what the magic does, while keeping the magic magical. Kindof like how all of Superman's powers can be explained by an ability to change the momentum of matter.

In the case of hard light, we could propose that magic is changing the vacuum energy in nearby space. The energy of the vacuum is increased and then guided into a spontaneous symmetry breaking. We get a new vacuum state where the Higgs field now couples to the photons, giving them mass. This vacuum, having a higher energy than our normal vacuum, is unstable and decays into normal vacuum when the magic is paused or the light travels outside the magicians's sphere of influence.

Scientifically speaking, the magician is creating a local false vacuum. Simply speaking, the laws of nature have changed in some small area around the magician.

WARNING: Care should be taken when altering the vacuum. If a vacuum with a lower energy state is created, it can cause a cascade effect where the rest of our universe drops into the same low vacuum state. The laws of nature in this new vacuum can be very different from our own. There may even exist vacuum states where (gasp!) magic no longer works!


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